The present invention relates generally to capacitive touch screen sensing systems, and more particularly to providing capacitive touch screen scanning circuitry which generates substantially less noise than the prior art, provides faster scanning than the prior art, and can operate from lower supply voltage than the prior art.
Touch sensing and multi-touch sensing displays are now widely used. There are a variety of approaches to touch sensing, and capacitive touch sensing is especially advantageous because it allows use of a hard touch surface (usually glass), and provides good performance.
FIG. 1 shows a section view of a typical capacitive touch sensing screen 1, which includes a dielectric layer 2 that may be the top layer of a conventional LCD display. An optional conductive shield layer 3 may be provided on dielectric layer 2 to prevent signals in the LCD display from inducing electrical noise on column grid conductors and/or row grid conductors of capacitive touch sensing screen 1. An intermediate dielectric layer 4 is provided on shield layer 3. A number of column grid conductors 5-1, . . . n are disposed on a upper surface of dielectric layer 4. Another dielectric layer 6 is disposed on column grid conductors 5-1,2 . . . q. A number of row grid conductors 7, only one of which appears in the section view of FIG. 1, are disposed on dielectric layer 6. A top dielectric layer 8, typically composed of glass, is disposed on row conductors 7. Row conductors 7 and column conductors 5-1,2 . . . q are normally formed from indium tin oxide (ITO) or other transparent conducting materials. The dielectric layers may be composed of glass or other insulating material. The tip of a finger 9 of a user is shown on a touch point 10 on the top surface of dielectric layer 8. As the tip of finger 9 is moved across the surface of dielectric layer 8, the cross-coupling capacitance, i.e., mutual capacitance, capacitance from various row conductors 7-1,2 . . . n to various column conductors 5-1,2 . . . q changes accordingly.
The presence of a finger touch on touch point 10 changes the capacitive coupling to and between row conductors 7 and column conductors 5-1,2 . . . q. This is because finger 9 is very resistive, and therefore the capacitive coupling from a column conductor 5-1,2 . . . q up to and through finger 9 and back down to various adjacent column conductors 5-1,2 . . . q, and also to various nearby row conductors 7-1,2 . . . n varies according to how the tip of finger 9 is pressed against the surface of dielectric layer 8. The coupling capacitances are very small, typically in the femto-farad range, and it is difficult to accurately sense the very low levels of capacitance change and resulting signal change on the various row conductors and column conductors.
Prior Art FIG. 2 schematically illustrates a conventional capacitive touch screen system 15-1 including drivers 16-1,2 . . . n which receive sequential row scanning control signals on conductors 17-1,2 . . . n, respectively, and capacitively couple corresponding signals to row conductors 7-1,2 . . . n of a capacitive touch screen device 1 such as the one shown in Prior Art FIG. 1. The row drive signals on conductors 7-1,2 . . . n are capacitively coupled, i.e., cross-coupled, to column conductors 5-1,2 . . . q. The capacitively cross-coupled signals on column conductors 5-1, 2 . . . q are applied to the inverting (−) inputs of sense amplifiers 21-1,2 . . . q, respectively. Sense amplifiers 21-1,2 . . . q have feedback capacitors 25-1,2 . . . q, respectively.
Capacitively coupled touch sensing is commonly used in touch screens (such as those in “smart phones” and the like) by using detection of touch-induced changes in the capacitances associated with row conductors 7-1,2 . . . n and column conductors 5-1,2 . . . q in order to determine locations of finger touch position 10 on the touch screen.
In the example shown in Prior Art FIG. 2, row excitation or scanning signals applied to the inputs of row drivers 16-1,2 . . . n are applied in a sequential or “round robin” fashion to excite row conductors 7-1,2 . . . n. The resulting corresponding capacitively coupled signals which appear on column conductors 5-1,2 . . . q are sensed by sense amplifiers 21-1,2 . . . q. Sinusoidal burst waveforms are typically used as the round robin excitation signals to avoid generating broadband interference on the touch sensor display. With knowledge of which of row conductors 7-1,2 . . . n was excited and which of column conductors 5-1,2 . . . q indicate associated capacitive signal coupling as the round robin scanning progresses, the position of a finger press on dielectric layer 8 can be determined.
However, a substantial challenge in obtaining an accurate computation of the touch point location is achieving sufficient signal power at the sense amplifier outputs 23-1,2 . . . q to allow accurate determination of the touch point location. Another substantial challenge is achieving sufficiently fast scanning of the entire array to allow finger touch motion to be tracked and to allow new finger touch points to be accurately determined and rapidly displayed.
These challenges exist partly because the use of low-cost driver circuits limits the magnitude of the drive signals. This is problematic because the amounts of capacitive coupling are so small that a large amount of signal processing is required to extract the desired signal out of all the noise and other incursions that are normally present. The limited magnitude of the drive signals is also problematic because the touch screen must be scanned rapidly in order to track the position and/or movement of the user's finger tip (or tips) sufficiently rapidly to have the corresponding events displayed on a LCD display under layer 2 (FIG. 1) to appear to be instantaneously responsive to the user. This is especially true if there are a large number of row conductors 7-1,2 . . . q to be scanned and if a very rapid display update rate is required.
Accordingly, the duration of each of the previously mentioned sinusoidal bursts needs to be short, and the amount of required subsequent signal processing is large. For example, suppose a complete scan of the entire touch screen is required every 10 milliseconds to accurately track the finger presses such that the user perceives instantaneous tracking of finger movement. That available 10 milliseconds must be divided by the number of rows of the touch screen assembly to determine how much time is available for each segment of the round robin sequence.
The required supply voltages for the above mentioned known touch screens typically are 5 to 12 volts. Use of the required high voltage levels for row drivers 16-1,2 . . . n in Prior Art FIG. 2 limits the process technologies and circuit design techniques that can be used.
The only practical options for increasing the sensitivity of the known touch screens are to increase the voltage of the drive amplifiers and/or reduce the round robin scanning rate. Unfortunately, the most readily available appropriate integrated circuit manufacturing processes may not permit the increased voltage needed for the driver circuits. Furthermore, fast touch screen response is required by the market in many applications. Consequently, it would be highly desirable to have a way of increasing the sensitivity and display update rate of a touch screen to a finger touch without relying on the above mentioned options.
The state-of-the-art is generally indicated in U.S. Pat. No. 7,663,607 entitled “Multipoint Touchscreen” issued Feb. 16, 2010 and U.S. Pat. No. 7,692,638 entitled “Error Compensation for Multi-Touch Surfaces”, issued Apr. 6, 2010.
When higher signal voltage and signal power levels are required in touch screen integrated circuitry to achieve the desired touch screen performance, more costly, higher voltage integrated circuit fabrication processes are required. It therefore becomes more difficult to integrate such circuitry with lower voltage, higher density, and lower cost circuitry that is ordinarily utilized for fabricating state-of-the-art integrated circuit touch screen control circuitry.
Known CDMA (code division multiple access) techniques in wireless systems involve use of orthogonal signals for transmitting between multiple base stations and handsets within a cell of wireless systems. Such conventional CDMA techniques involve transmitting multiple signals in a single channel and attempting to separate the information content in the individual channels while trying to keep the multiple signals separate in the single channel.
There is an unmet need for a capacitively coupled touch screen system having increased touch sensitivity without requiring increased supply voltage levels and without requiring use of a reduced “round robin” scanning rate.
There also is an unmet need for a capacitively coupled touch screen system which has fundamentally higher SNR (signal to noise ratio), faster touch screen response, and the ability to operate at lower power/voltage levels than the closest prior art capacitively coupled touch screen systems.
There also is an unmet need for a capacitively coupled touch screen system which allows “lower-voltage”, less costly semiconductor wafer fabrication processes to be utilized in the manufacture of faster, more accurate capacitively coupled touch screen systems.
There also is an unmet need for a capacitively coupled touch screen system which allows more accurate determination of all of the cross-coupling coefficients between grid conductors of the capacitive sensing touch screen.